1. Field of the Invention
This invention relates to the production of purified phosphoric acid and, in particular, to the production of concentrated high-purity phosphoric acid.
2. The Prior Art
Most of the high-purity phosphoric acid on the market today is produced by the so-called furnace process, which involves the production of elemental phosphorus in an electric furnace from phosphate rock and coal. The elemental phosphorus is then burned and the resulting phosphorus pentoxide is hydrolyzed to high purity phosphoric acid. This technology is generally costly and very energy intensive. Efforts have been made in the past to develop technology for the production of high-purity phosphoric acid from impure acids, such as wet-process acid. Wet-process acid is produced via the acidulation of phosphate rock with sulfuric acid, and is less expensive to make. Such acid, however, is contaminated with significant concentrations of numerous impurities, such as iron, aluminum, magnesium, sulfate, fluorine and silica. Other impure acids with similar impurities are available "spent acids", that is, acids which, regardless of their original manufacture or purity, e.g., furnace process or wet-process, have been used ("spent") in such industrial applications as metal finishing or in catalyst applications.
While crystallization of the phosphoric acid would normally be considered as a process which would result in a crystallized product of relatively high purity (leaving behind a raffinate containing the rejected impurities), crystallization has not been practiced on an industrial scale for purifying wet-process acid, or for purifying other impure acids. Apparently crystallization has not been commercially accepted because of great difficulty in controlling the rate of crystallization. When the impurities normally associated with wet-process or spent acids are present, the impure acid can withstand very substantial cooling, well into the supersaturation region, before crystallization occurs. Even then spontaneous crystallization can be an extremely slow process. However, once crystals are formed by spontaneous (primary) nucleation, or if seed crystals are added in amounts substantially lower than the amounts used in the method of this invention, the impure acid tends to crystallize relatively rapidly (presumably by secondary nucleation) to a putty-like intractable mass which has a viscosity typically in excess of 50,000 centipoises and which cannot be further processed or separated. This rapid crystallization of phosphoric acid into a putty-like intractable mass is hereinafter referred to as "catastrophic crystallization".
A number of processes have been proposed for removing impurities from phosphoric acid by either extraction or crystallization. For example, U.S. Pat. No. 3,642,439 describes an attempt to provide a process for upgrading the purity of wet-process phosphoric acid. In this process the inventors claim that magnesium can be selectively removed from the wet-process acid via the crystallization of magnesium-containing precipitates. The examples cited in the '439 patent indicate that the efficiency of the process is very limited. The magnesium content before the precipitation step in one of the examples was 0.4%, while after the crystallization and filtration the magnesium content of the purified acid was 0.2%. Thus, the process facilitates only the removal of about 50% of the magnesium content in the feed acid and does practically nothing to remove other impurities contained in the wet-process phosphoric acid.
U.S. Pat. No. 4,299,804 describes another process for the removal of impurities from wet-process phosphoric acid by crystallization. In this case magnesium and aluminum impurities are claimed to be removed in the form of a magnesium-aluminum fluoride. Magnesium removal efficiencies of up to 90% are indicated by the examples; however, aluminum removal effectiveness is generally much poorer and the product still contains the other impurities such as iron, sodium, silica and fluoride. The examples indicate that the efficiency of the aluminum and magnesium removal process varies from sample to sample.
U.S. Pat. No. 4,243,643 refers to another process for the removal of metallic ion impurities from wet-process phosphoric acid. This process also suffers from several distinct disadvantages. It requires the use of a precipitant comprising ions of calcium and fluorine to cause the precipitation of magnesium from the acid, and it requires that the sulfate concentration of the acid exceed 2%. Even then the effectiveness of the process is only of the order of about 50% for magnesium and even lower with respect to other metallic impurities present in the wet-process acid, such as iron, aluminum and sodium.
U.S. Pat. No. 3,890,097 concerns a process for the purification of wet-process phosphoric acid which involves the crystallization from wet-process acid of a P.sub.2 O.sub.5 -containing entity rather than of the impurities. This patent suggests the addition of a quantity of sulfuric acid to wet-process phosphoric acid in an amount sufficient to raise the concentration of sulfuric acid in the solution to a range of from about 10% to 15% by weight. The '097 patent points out that crystallization of wet-process acid is impractical because of the low temperatures required and the high viscosities which occur. The addition of sulfuric acid to the impure phosphoric acid is claimed to lower its viscosity and increase its freezing point. The distinct disadvantage of this process lies in the need for the addition of costly sulfuric acid which is used to modify the physical characteristics, specifically the freezing point and the viscosity, of the phosphoric acid solution from which the purified material is crystallized. As a consequence of this sulfuric acid addition, the sulfuric acid content of the purified phosphoric acid is relatively high, that is, over 1% by weight, and the process is further burdened by a higher water content in the raffinate which carries about 50% of the original P.sub.2 O.sub.5 values.
British Patent No. 1,436,115 also makes reference to crystallization in purifying wet-process phosphoric acid. In this patent, however, the need to first purify the wet-process acid by solvent extraction is stressed. The disclosure teaches that it is not in fact practicable to produce a purified phosphoric acid by direct crystallization from wet-process phosphoric acid. A similar opinion is expressed in U.S. Pat. No. 3,912,803.
U.S. Pat. Nos. 4,215,098 and 4,296,082 teach that crystallization of phosphoric acid is to be preceded by a purification step and offer heat treatment processes which serve to bring the phosphoric acid to a concentration about 76% P.sub.2 O.sub.5 and precipitate dissolved impurities from the acid. Only then is the acid diluted and subjected to crystallization.
U.S. Pat. No. 4,083,934 discloses a process for obtaining purified crystallized orthophosphoric acid from superphosphoric acid. The patent does not address the direct purification of wet-process phosphoric acid or the crystallization of phosphoric acid hemihydrate.
Japanese Patent No. 14,692, published in 1969, describes a process for purifying phosphoric acid by crystallization. In this patent the patentees point out that, although crystallization would be a desirable method for purifying phosphoric acid, it has not been employed industrially. Working from the assumption that it is the impurities which adversely affect the rate of crystallization, the Japanese patent describes a pre-crystallization process using oxidants which remove not only organic impurities but also inorganic impurities, such as calcium phosphate, calcium sulfate, chromium, vanadium and manganese, followed by further pre-processing to remove fluoride impurities. It is only after such pre-purification, according to this patent, that practical crystallization can be employed.
In the Proceedings of a Conference of Industrial Crystallization, published in 1976, Aoyama and Toyokura describe a process said to bring about crystallization of phosphoric acid from crude wet-process acid concentrated to about 60% P.sub.2 O.sub.5. Although the authors claim to have operated a pilot-scale crystallizer for as much as two weeks satisfactorily, nothing is said in the description as to conditions of seeding or control which would preclude catastrophic crystallization. As discussed below in the description of the present invention, it is the problem of catastrophic crystallization which the present invention overcomes by proper control of the seeding conditions. The only discussion of seeding in the Aoyama et al. paper refers to control of the circulation rates through different sections of the crystallizer, which are said to affect the number of seed crystals in the growing bed. However, the details of this control are not described. To the extent it is indicated in their process description it appears that the "seed" crystals are, in fact, products of primary crystallization of the wet-process phosphoric acid. In developing the method of the instant invention the inventors have found that such crystals, when used as the sole source of seed, do not provide controllable results.
An object of this invention is to provide a method for controlling the seeding and other conditions required to avoid catastrophic crystallization while crystallizing phosphoric acid. Another object is to provide a method for purifying phosphoric acid by means of crystallization without the need of solvent extraction techniques. Another object is to provide a process for the manufacture of high-purity phosphoric acid from wet-process phosphoric acid by the selective crystallization of phosphoric acid hemihydrate crystals from the impurities normally associated with the wet-process acid. Still another object of the present invention is to provide a method for purifying wet-process phosphoric acid by crystallization of its P.sub.2 O.sub.5 entity without the continuous use of reagent additives. A further object of the invention is to provide a method for producing a purified phosphoric acid having a higher P.sub.2 O.sub.5 concentration than the feed acid from which it is made. A still further object is to provide a process for manufacturing concentrated high purity phosphoric acid from wet-process phosphoric acid by means of selective crystallization of the P.sub.2 O.sub.5 entity in the wet-process acid which process affords the flexibility of simultaneously manufacturing various purity grades of concentrated phosphoric acid products by means of remelting and recrystallizing said products.